The present invention is based on a steering wheel skeleton, which is produced in one piece from an aluminum alloy of the Alxe2x80x94Mgxe2x80x94Mn type by die casting.
European Patent No. 412 605 describes a steering wheel skeleton, where the steering wheel hub, steering wheel spokes and the encapsulation of the steering wheel spokes on the steering wheel rim are die cast in one piece from an aluminum alloy. The proposed materials for use here are hardenable aluminum alloys of the AlSi7Mg, AlSi9Mg or AlSi10Mg type enriched with antimony or strontium. To increase its elongation at break, the steering wheel skeleton is subjected to a conventional hardening treatment, i.e., a heat treatment in the form of solution annealing, quenching and subsequent elevated-temperature age hardening.
It is also known from European Patent No. 412 605 that the heat treatment described above can be omitted if an aluminum alloy of the Alxe2x80x94Mgxe2x80x94Mn type is used instead of the above-mentioned Alxe2x80x94Sixe2x80x94Mg alloys as the material for die casting the steering wheel skeleton. Steering wheel skeletons of Alxe2x80x94Mgxe2x80x94Mn alloys even in the cast state should have the mechanical properties, in particular a high elongation at break, that are most important for operating reliability and accident safety, with a suitable deformation stress even under accident conditions.
However, starting from these casting materials, there is still a need for a further increase in ductility of the steering wheel skeleton for safety steering wheels under accident conditions in order to be able to ensure even greater deformation of the steering wheel skeleton without breakage of the steering wheel spokes. This concerns in particular use in steering wheel designs where the steering wheel rim is greatly offset axially with respect to the steering wheel hub, and the steering wheel spokes may be subject to severe compression under accident conditions. In addition, this concerns in particular steering wheel skeletons where the hub, spokes and steering wheel rim may be cast in one piece to reduce the weak points within the skeleton and for technical manufacturing reasons, and therefore they are made of the same material, which is why especially high demands are made of this material with regard to strength and workability.
Therefore, an object of the present invention is to further improve upon one-piece die-cast steering wheel skeletons made of aluminum alloys of the Alxe2x80x94Mgxe2x80x94Mn type by increasing their ductility, so that the steering wheel skeleton will preferably undergo only compression but no breakage even under a high acting force.
Then the cast steering wheel skeleton made of an aluminum alloy of the Alxe2x80x94Mgxe2x80x94Mn group is subjected as a whole to a heat treatment, preferably by first heating the steering wheel skeleton to a temperature between 250xc2x0 C. and 400xc2x0 C. This heating corresponds to an averaging treatment to achieve an equilibrium of the hardenable elements. The heating time is preferably between one hour and four hours. Then the steering wheel skeleton is cooled in stationary air.
The steering wheel skeletons heat treated in this way have a greatly increased ductility. The compression length by which a heat-treated steering wheel skeleton can be deformed under the influence of a static pressure without breakage of the spokes is almost twice as great as the compression length of the untreated steering wheel skeleton in the cast state. Thus, a very high compression of the steering wheel spokes without breakage can be achieved even with steering wheel designs where the steering wheel rim is offset axially to a great extent with respect to the steering wheel hub.
Heating of the steering wheel skeleton as a whole and cooling in stationary air guarantee that there is only negligible thermal deformation in comparison with the cast state due to the heat treatment in the steering wheel skeleton. These heat-treated steering wheel skeletons can therefore be processed further without any post-straightening despite the high accuracy demands made of them.